Emissions control systems for vehicles include a fuel vapor canister configured to adsorb refueling, diurnal, and running loss vapors. Vehicles classified as practically zero emissions vehicles (PZEVs) in North America are required to include a bleed canister in addition to the main fuel vapor canister. Over the course of one or more diurnal cycles, fuel vapor may become desorbed from the main fuel vapor canister. The bleed canister may be located between the main fuel vapor canister and atmosphere and configured to bind the desorbed vapors.
In order to prevent bleed emissions, the bleed canister may include honeycomb shaped folds of adsorbent or other such structures to provide a large surface area for binding low weight hydrocarbons. However, this may cause airflow through the canister to be highly restrictive. For systems where the bleed canister is in series with the main fuel vapor canister, the bleed canister may effectively act as an airflow resistor that reduces flow through the main canister. Similarly, the main canister may act as a resistor that reduces flow through the bleed canister.
During a purge event, this may increase the time needed to purge the canisters. For hybrid vehicles, and other vehicles with limited engine run time and/or limited engine intake vacuum, the opportunities to purge the canisters may already be limited. Failure to purge the canisters may lead to bleed emissions or failed evaporative emissions tests. If the bleed canister is empty, there is no practical reason to direct purge air there through. If the bleed canister is loaded, passing purged vapors into the main canister before directing them to intake may reduce the purge efficiency. Further, during refueling events, refueling vapors may be generated at a higher rate than air stripped of the vapors exits to atmosphere if it must pass through the restrictive bleed canister. This may lead to pre-mature shutoffs of the refueling dispenser.
The inventors herein have recognized the above issues, and have developed systems and methods to at least partially address them. In one example, a method for a fuel system is presented, comprising directing purge air through a first fuel vapor canister but not a second fuel vapor canister based on a load of the first fuel vapor canister being greater than a threshold. In this way, purging of the first canister may be prioritized and completed without passing desorbed fuel vapors through the second canister, thereby reducing the amount of time needed to purge the first canister, and reducing overall emissions.
In another example, a method for a fuel system is presented, comprising: directing fuel vapor from a fuel tank into a first fuel vapor canister but not a second fuel vapor canister based on a fuel tank pressure. In this way, fuel tank depressurization may proceed more rapidly than it would if the fuel vapor were directed through a more restrictive pathway. Further, fuel tank pressure during refueling may be moderated, thus preventing automatic shutoffs of the refueling pump.
In yet another example, an emissions control system for a vehicle is presented, comprising: a fuel tank coupled to a main fuel vapor canister via a fuel tank isolation valve, a canister purge valve coupled between the main fuel vapor canister and an engine intake, a bleed fuel vapor canister coupled between the main fuel vapor canister and atmosphere, a bleed canister bypass valve coupled within a bleed canister bypass conduit, and a controller configured with instructions stored in non-transitory memory, that when executed, cause the controller to: open the canister purge valve; and open the bleed canister bypass valve responsive to a load of the bleed fuel vapor canister being less than a threshold. In this way, if the bleed canister is not loaded with hydrocarbons, it may be bypassed, thus increasing the purge flow through the main fuel vapor canister.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.